Stainless steel for oil well, stainless steel pipe for oil well, and method of manufacturing stainless steel for oil well

ABSTRACT

The chemical composition of a stainless steel in accordance with the present invention consists of C: not more than 0.05%, Si: not more than 0.5%, Mn: 0.01 to 0.5%, P: not more than 0.04%, S: not more than 0.01%, Cr: more than 16.0 and not more than 18.0%, Ni: more than 4.0 and not more than 5.6%, Mo: 1.6 to 4.0%, Cu: 1.5 to 3.0%, Al: 0.001 to 0.10%, and N: not more than 0.050%, the balance being Fe and impurities, and satisfies Formulas (1) and (2). Also, the micro-structure thereof contains a martensitic phase and a ferritic phase having a volume ratio of 10 to 40%, and the ferritic phase distribution ratio is higher than 85%. 
       Cr+Cu+Ni+Mo 25.5  (1)
 
       −8 ≦30(C+N)+0.5Mn+Ni+Cu/2+8.2−1.1(Cr +Mo) ≦−4  (2)

TECHNICAL FIELD

The present invention relates to a stainless steel for oil well and astainless steel pipe for oil well. More particularly, the presentinvention relates to a stainless steel for oil well and a stainlesssteel pipe for oil well, which are used in a high-temperature oil wellenvironment and gas well environment (hereinafter, referred to as “ahigh-temperature environment”).

BACKGROUND ART

Recently, the development of oil wells and gas wells in deep layer hasbeen advanced. (hereinafter, an oil well and a gas well are collectivelyreferred simply to as “an oil well”. Also herein, “a stainless steel foroil well” includes a stainless steel for oil well and a stainless steelfor gas well, and “a stainless steel pipe for oil well” includes astainless steel pipe for oil well and a stainless steel pipe for gaswell.) A deep oil well has a high-temperature environment. “Ahigh-temperature environment” contains carbon dioxide gas or carbondioxide gas and hydrogen sulfide gas, which are corrosive gases. Theterm “a high temperature” as used herein represents a temperature notlower than 150 degrees C. The oil well pipe used in a high-temperatureenvironment of deep oil well is required to meet the three requirementsas below.

(1) High strength. Specifically, the 0.2% offset yield stress is 758 MPaor higher (110 ksi class or higher). For the deep oil well, since thewell has a large well depth, the length and weight of steel pipe usedincrease. Therefore, a high strength is required.

(2) Excellent corrosion resistance. Specifically, the corrosion rate ina high-temperature environment is lower than 0.1 g/(m²+hr). Further, theoil well pipe is less liable to crack even when the pipe is stressed.That is, the oil well pipe has excellent stress corrosion crackingresistance. Hereinafter, “stress corrosion cracking” is also abbreviatedas SCC. When reference is made to “excellent corrosion resistance inhigh-temperature environment” herein, it means that the corrosion rateis low, and the SCC resistance is excellent.

(3) Excellent sulfide stress corrosion cracking resistance at normaltemperature. In the case where the stainless steel pipe for oil well isused for a production well, a fluid (oil or gas) produced from the oilwell in high-temperature environment flows in the stainless steel pipe.When the production of fluid from the oil well stops for some reason,the temperature of the fluid in the stainless steel pipe near theearth's surface decreases to the normal temperature. At this time,sulfide stress corrosion cracking (hereinafter, also abbreviated as SSC)may occur in the stainless steel pipe that is in contact with thenormal-temperature fluid. Therefore, the stainless steel pipe for oilwell is required to have not only SCC resistance at high temperaturesbut also SSC resistance at normal temperature.

JP2005-336595A (hereinafter, referred to as Patent Document 1),JP2006-16637A (hereinafter, referred to as Patent Document 2), andJP2007-332442A (hereinafter, referred to as Patent Document 3) haveproposed stainless steels for the use in high-temperature environments.In improving the corrosion resistance at high-temperature environments,chromium (Cr) is effective. Therefore, the stainless steels disclosed inPatent Documents 1 to 3 contain much Cr.

The stainless steel pipe disclosed in Patent Document 1 contains 15.5 to18% of Cr, this Cr content being higher than that of the conventionalmartensitic stainless steel (the Cr content is 13%). Further, thechemical composition of the stainless steel pipe satisfies the formulaof Cr+Mo+0.3Si−43.5C−0.4 Mn−Ni−0.3Cu−9N≧11.5. Since the chemicalcomposition satisfies this formula, the micro-structure consists of atwo-phase micro-structure of ferritic phase and martensitic phase. As aresult, the hot workability is improved. Further, the chemicalcomposition of the stainless steel pipe contains Ni and Mo as essentialelements and contains Cu as a selective element. Therefore, thecorrosion resistance of stainless steel pipe is improved.

The stainless steel pipe disclosed in Patent Document 2 contains 15.5 to18.5% of Cr. Further, the stainless steel disclosed in Patent Document 2contains Ni, which improves the corrosion resistance, as an essentialelement. In the stainless steel pipe disclosed in Patent Document 2, Moand Cu are selective elements.

The stainless steel pipe disclosed in Patent Document 3 contains 14 to18% of Cr. The stainless steel pipe disclosed in Patent Document 3further contains Ni, Mo and Cu. Therefore, the stainless steel pipe iscorrosion resistant. Further, the micro-structure of the stainless steelpipe disclosed in Patent Document 3 contains a martensitic phase and anaustenitic phase having a volume ratio of 3 to 15%. Therefore, thestainless steel pipe is tough.

As described above, the stainless steels disclosed in Patent Documents 1to 3 contain more than 13% of Cr. Further, these stainless steelscontain alloying elements of Ni, Mo, Cu, etc. as an essential element ora selective element. Therefore, the corrosion rate in high-temperatureenvironments decreases. For example, in the working example of PatentDocument 1, a decrease in corrosion rate in high-temperatureenvironments has been proved (refer to Table 2 in Patent Document 1).

DISCLOSURE OF THE INVENTION

Unfortunately, in the stainless steel pipes disclosed in PatentDocuments 1 to 3, cracking may occur when a stress is applied in ahigh-temperature environment. That is, stress corrosion cracking mayoccur in a high-temperature environment. Therefore, the stainless steelsdisclosed in Patent Documents 1 to 3 may not meet the above-describedrequirements (1) to (3).

Accordingly, an object of the present invention is to provide astainless steel for oil well having the following properties:

high strength, specifically, a 0.2% offset yield stress not lower than758 MPa;

excellent corrosion resistance in high-temperature environments; and

excellent SSC resistance at normal temperature.

The inventors conducted studies and found that the stainless steel thatmeets the items (A) to (C) below can satisfy the above-describedrequirements (1) to (3).

(A) The Cr content is higher than 16.0% by mass percent. Further, Cr,Ni, Cu and Mo are contained so as to satisfy the following formula:

Cr+Cu+Ni+Mo≧25.5  (1)

where the content (mass%) of element is substituted for thecorresponding symbol of element in the formula.

If the Cr content is increased, and Formula (1) is satisfied, a strongpassivation film is formed on the steel surface in high-temperatureenvironments. Therefore, the corrosion resistance is improved. Morespecifically, the corrosion rate in high-temperature environments isdecreased, and the SCC resistance is improved.

(B) The micro-structure contains a martensitic phase and a ferriticphase having a volume ratio of 10 to 40%. Further, the ferritic phasedistribution ratio should be higher than 85%. The ferritic phasedistribution ratio is explained below.

FIG. 1 is a photograph of a cross section near the surface of astainless steel in accordance with the present invention. Referring toFIG. 1, a plurality of ferritic phases 5 extend along a surface 1 of thestainless steel. Almost all of portions other than the ferritic phases 5in the cross section are a martensitic phase 6.

The ferritic phase distribution ratio is a measure indicating the mannerof ferritic phases distributed in a portion near the surface. Theferritic phase distribution ratio is defined as described below. Asshown in FIG. 2, a scale 10 having a length of 200 μm is prepared. Inthe scale 10, a plurality of imaginary line segments 20 each having alength of 50 μm are arranged in a row at intervals of 10 μm over therange of 200 μm in the longitudinal direction of the scale 10. The scale10 is placed so that the upper side of the scale 10 coincides with thesurface 1 of the stainless steel shown in FIG. 1. FIG. 3 shows aphotograph in which the scale 10 is applied. Each of the imaginary linesegments 20 has a length of 50 μm in the thickness direction ofstainless steel from the surface 1. The plurality of imaginary linesegments 20 are arranged in a row at intervals of 10 μm over the rangeof 200 μm along the surface of stainless steel. When the scale 10 isplaced on the cross section of stainless steel as shown in FIG. 3, theferritic phase distribution ratio (%) is defined by the followingformula (a):

Ferritic phase distribution ratio=number of imaginary line segmentscrossing ferritic phases/total number of imaginary line segments×100 (a)

In short, the ratio of the number of imaginary line segments crossingferritic phases to the total number of imaginary line segments isdefined as the ferritic phase distribution ratio (%). As describedabove, the ferritic phase distribution ratio is higher than 85%. If theferritic phase distribution ratio is higher than 85%, the SCC resistancein high-temperature environments is improved. FIG. 4 is a photograph ofa cross section of a stainless steel having a ferritic phasedistribution ratio of 71.4%. As shown in FIG. 4, a crack 7 produced inthe surface 1 is propagated in the thickness direction of the stainlesssteel. When the front edge of the crack 7 reaches a ferritic phase 5,the propagation of the crack 7 stops. That is, the ferritic phase 5inhibits the propagation of crack. In FIG. 4, since the ferritic phasedistribution ratio is not higher than 85%, the ferritic phases 5 are notdistributed widely in a portion near the surface (that is, a depth rangeof 50 from the surface). Therefore, the crack 7 is propagated to somedepth.

In contrast, the ferritic phase distribution ratio of the stainlesssteel shown in FIG. 1 is higher than 85%. That is, the ferritic phases 5are distributed widely in a portion near the surface. Therefore, when acrack is produced in the surface 1, the crack reaches a ferritic phaseat a position shallow from the surface 1, and the propagation thereofstops. Therefore, the SCC resistance in high-temperature environments isimproved.

(C) Copper (Cu) is contained in large amounts as an essential element.Specifically, the Cu content should be 1.5 to 3.0% by mass percent. In ahigh-temperature environment, Cu restrains the propagation of cracks.Therefore, the SCC resistance in high-temperature environments isimproved. The mechanism of this is assumed as described below. If the Cucontent is 1.5 to 3.0%, a passivation film is likely to form on thesurface of a crack that stops propagating at a ferritic phase.Therefore, new stress corrosion cracking can be restrained fromoccurring from the crack surface.

Based on the above-described knowledge, the inventors completed aninvention described below.

The stainless steel for oil well in accordance with the presentinvention has a chemical composition and micro-structure describedbelow, and has a 0.2% offset yield stress not lower than 758 MPa. Thechemical composition thereof consists of, by mass percent, C: 0.05% orless, Si: 0.5% or less, Mn: 0.01 to 0.5%, P: 0.04% or less, S: 0.01% orless, Cr: more than 16.0 and not more than 18.0%, Ni: more than 4.0 andnot more than 5.6%, Mo: 1.6 to 4.0%, Cu: 1.5 to 3.0%, Al: 0.001 to0.10%, and N: 0.050% or less, the balance being Fe and impurities, andsatisfies Formulas (1) and (2). The micro-structure thereof contains amartensitic phase and a ferritic phase having a volume ratio of 10 to40%. When a plurality of imaginary line segments, which each have alength of 50 μm in the thickness direction from the surface of stainlesssteel and are arranged in a row at intervals of 10 μm over the range of200 μm, are placed on the cross section of the stainless steel, theratio of the number of imaginary line segments crossing ferritic phasesto the total number of imaginary line segments is higher than 85%.

Cr+Cu+Ni+Mo≧25.5  (b 1)

−8≦30(C+N)+0.5 Mn+Ni+Cu/2+8.2−1.1 (Cr+Mo)≦−4 (2)  (2)

where the content (mass %) of element is substituted for the symbol ofelement in Formulas (1) and (2).

The 0.2% offset yield stress is defined as described below. In astress-strain curve graph in which the ordinates represent stress andthe abscissas represent strain, a stress corresponding to theintersection of the stress-strain curve and an imaginary straight linein parallel with a straight-line portion (elastic zone) of the curve isreferred to as an offset yield stress. The distance between the startingpoint of the stress-strain curve and the point at which the imaginarystraight line intersects with the abscissa is referred to as an offsetamount. An offset yield stress having an offset amount of 0.2% isreferred to as a 0.2% offset yield stress.

Preferably, the aforementioned chemical composition contains, in placeof some of Fe, one or more kinds selected from the group consisting ofV: 0.25% or less, Nb: 0.25% or less, Ti: 0.25% or less, and Zr: 0.25% orless.

Preferably, the above-described chemical composition contains, in placeof some of Fe, one or more kinds selected from the group consisting ofCa: 0.005% or less, Mg: 0.005% or less, La: 0.005% or less, and Ce:0.005% or less.

Preferably, the aforementioned micro-structure contains a retainedaustenitic phase having a volume ratio not more than 10%.

A stainless steel pipe for oil well in accordance with the presentinvention is manufactured by using the above-described stainless steel.

A method of manufacturing a stainless steel for oil well in accordancewith the present invention includes the following steps of S1 to S4:

(S1) A step of heating a steel stock having a chemical compositionconsisting of, by mass percent, C: 0.05% or less, Si: 0.5% or less, Mn:0.01 to 0.5%, P: 0.04% or less, S: 0.01% or less, Cr: more than 16.0 andnot more than 18.0%, Ni: more than 4.0 and not more than 5.6%, Mo: 1.6to 4.0%, Cu: 1.5 to 3.0%, Al: 0.001 to 0.10%, and N: 0.050% or less, thebalance being Fe and impurities, and satisfying Formulas (1) and (2).

(S2) A step of hot working the steel stock so that the reduction of areaof the steel stock at a steel stock temperature of 850 to 1250° C. isnot less than 50%.

(S3) A step of heating the steel stock to a temperature not lower thanAc3 transformation point and quenching it after the hot working.

(S4) A step of tempering the steel stock at a temperature not higherthan Ac1 transformation point after the quenching.

The reduction of area (%) is defined by the following formula (3):

Reduction of area=(1−steel stock cross-sectional area perpendicular tothe steel stock longitudinal direction after hot working/steel stockcross-sectional area perpendicular to the steel stock longitudinaldirection before hot working)×100  (3)

Through the above-described steps, a stainless steel for oil well,having the above-described chemical composition, micro-structure, andyield stress, is manufactured.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph of a cross section of a stainless steel for oilwell in accordance with the present invention;

FIG. 2 is a view showing a scale for measuring a ferritic phasedistribution ratio;

FIG. 3 is a view for explaining a method of measuring a ferritic phasedistribution ratio by using the scale shown in FIG. 2; and

FIG. 4 is a photograph of a cross section of a stainless steel having aferritic phase distribution ratio of 85% or lower.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will now be described in detail.

1. Chemical Composition

The stainless steel for oil well in accordance with the presentinvention has a chemical composition described below. Hereinafter, thepercentage relating to the element means mass percent.

C: 0.05% or less

Carbon (C) improves the strength of steel. However, if the C content istoo high, the hardness after tempering becomes excessively high, and theSSC resistance is deteriorated. Further, in the chemical composition ofthe present invention, as the C content increases, the Ms pointdecreases. Therefore, as the C content increases, the retained austeniteis liable to increase, and the 0.2% offset yield stress is liable todecrease. Therefore, the C content should be 0.05% or less. Thepreferable C content is 0.03% or less. The lower limit of C content isnot subject to any special restriction. However, considering the cost ofdecarburization in the steel making process, the preferable C content is0.003% or more, further preferably 0.007% or more.

Si: 0.5% or less

Silicon (Si) deoxidizes steel. If the Si content is too high, thetoughness and hot workability of steel are deteriorated. Therefore, theSi content should be 0.5% or less.

Mn: 0.01 to 0.5%

Manganese (Mn) deoxidizes and desulfurizes steel, and improves the hotworkability. If the Mn content is too low, the above-described effectscannot be achieved. If the Mn content is too high, the corrosionresistance in high-temperature environments is deteriorated. Therefore,the Mn content should be 0.01 to 0.5%. The preferable Mn content is0.05% or more and less than 0.2%.

P: 0.04% or less

Phosphorus (P) is an impurity. Phosphorus deteriorates the SSCresistance. Therefore, the P content should be 0.04% or less. Thepreferable P content is not more than 0.025%.

S: 0.01% or less

Sulfur (S) is an impurity. Sulfur deteriorates the hot workability.Therefore, the S content should be 0.01% or less. The preferable Scontent is not more than 0.005%, further preferably not more than0.002%.

Cr: more than 16.0 and not more than 18.0%

Chromium (Cr) improves the corrosion resistance in high-temperatureenvironments. Specifically, Cr decreases the corrosion rate inhigh-temperature environments and improves the SCC resistance. If the Crcontent is too low, the above-described effects cannot be achieved. Ifthe Cr content is too high, the ferritic phase in steel increases, andthe strength of steel is deteriorated. Therefore, the Cr content shouldbe more than 16.0% and not more than 18.0%. The preferable Cr content is16.3 to 18.0%.

Ni: more than 4.0 and not more than 5.6%

Nickel (Ni) improves the strength of steel. Further, Ni improves thecorrosion resistance in high-temperature environments. If the Ni contentis too low, the above-described effects cannot be achieved. However, ifthe Ni content is too high, the amount of retained austenite produced isliable to increase. Hereby, it is difficult to obtain a 0.2% offsetyield stress of 758 MPa or higher.

Therefore, the Ni content should be more than 4.0% and not more than5.6%. The preferable Ni content is 4.2 to 5.4%.

Mo: 1.6 to 4.0%

Molybdenum (Mo) improves the SSC resistance. If the Mo content is toolow, the above-described effect cannot be achieved. On the other hand,even if Mo is contained excessively, the above-described effectsaturates. Therefore, the Mo content should be 1.6 to 4.0%. Thepreferable Mo content is 1.8 to 3.3%.

Cu: 1.5 to 3.0%

Copper (Cu) improves the strength of steel by means of precipitationhardening. Further, as described above, Cu improves the SCC resistancein high-temperature environments. Still further, Cu decreases thecorrosion rate. If the Cu content is too low, the above-describedeffects cannot be achieved. If the Cu content is too high, the hotworkability is deteriorated. Therefore, the Cu content should be 1.5 to3.0%. The preferable Cu content is 2.0 to 3.0%, further preferably 2.3to 2.8%.

Al: 0.001 to 0.10%

Aluminum (Al) deoxidizes steel. If the Al content is too low, theabove-described effect cannot be achieved. If the Al content is toohigh, the inclusions in steel increase, so that the corrosion resistanceis deteriorated. Therefore, the Al content should be 0.001 to 0.10%.

N: 0.050% or less

Nitrogen (N) improves the strength of steel. However, if the N contentis too high, the inclusions in steel increase, so that the corrosionresistance is deteriorated. Therefore, the N content should be 0.050% orless. The preferable N content is 0.026% or less. The lower limit valueof preferable N content is 0.002%.

The chemical composition of the stainless steel in accordance with thepresent invention further satisfies Formula (1):

Cr+Cu+Ni+Mo≧25.5  (1)

where the content of element is substituted for the corresponding symbolof element in Formula (1).

If the contents of Cr, Cu, Ni and Mo in the steel satisfy Formula (1),in high-temperature environments, a strong passivation film is formed onthe surface of the stainless steel. Therefore, the corrosion rate inhigh-temperature environments decreases. Further, the SCC resistance isimproved in high-temperature environments.

2. Micro-structure

The stainless steel in accordance with the present invention has amicro-structure containing a ferritic phase having a volume ratio of 10to 40%. The remaining portion of micro-structure other than the ferriticphase is mainly a martensitic phase, additionally including a retainedaustenitic phase. If the amount of retained austenitic phase increasesexcessively, it is difficult to obtain high strength. Therefore, thepreferable volume ratio of retained austenitic phase in the steel is 10%or less.

The volume ratio of the ferritic phase is determined by the methoddescribed below. A sample may be taken from any location in thestainless steel. The sample surface corresponding to the cross sectionof the stainless steel is ground. After being ground, the ground samplesurface is etched by using a solution in which glycerin is mixed withaqua regia. Using an optical microscope (observation magnification×100),the area ratio of ferritic phase on the etched surface is measured bythe point counting method conforming to JISG0555. The measured arearatio is defined as the volume ratio of ferritic phase.

The volume ratio of the retained austenitic phase is determined by theX-ray diffraction method. A sample may be taken from any location in thestainless steel. The size of the sample is 15 mm×15 mm×2 mm. Using thissample, X-ray intensity is measured on the (200) plane of α (ferrite)phase, the (211) plane of a phase, and the (200) plane, the (220) plane,and the (311) plane of γ (retained austenite) phase. Then, theintegrated intensity on each plane is calculated. After the calculation,the volume ratio Vγ(%) is calculated for each of combinations of theplanes of a phase and the planes of γ phase (a total of sixcombinations) by using Formula (4). The mean value of the volume ratiosVγ of the six combinations is defined as the volume ratio (%) ofretained austenite.

Vγ=100/(1+(Iα.Rγ)/(IγRα))  (4)

where Iα is the integrated intensity of a phase, Rα is thecrystallographic theoretical calculation value of α phase, Iγ is theintegrated intensity of γ phase, and Rγ is the crystallographictheoretical calculation value of γ phase.

If the volume ratio of ferritic phase is 10 to 40%, a 0.2% offset yieldstress of 758 MPa or higher can be obtained. Further, the ferritic phaseinhibits the propagation of cracking. Therefore, the SCC resistance inhigh-temperature environments is improved.

The micro-structure of the stainless steel whose chemical compositionsatisfies Formula (2) and which is manufactured by the manufacturingmethod described later can have a configuration containing 10 to 40% offerritic phase.

−8≦30(C+N)+0.5Mn+Ni+Cu/2+8.2−1.1(Cr+Mo)≦−4  (2)

where the content of element is substituted for the corresponding symbolof element in Formula (2).

It is defined that X=30(C+N)+0.5Mn+Ni+Cu /2+8.2−1.1(Cr+Mo). If X is lessthan −8, the volume ratio of ferritic phase exceeds 40%. If the volumeratio of ferritic phase exceeds 40%, cracking is liable to occur inhigh-temperature environments. The reason for this is unclear; however,the reason can be assumed as described below. The concentrationdistribution of Cr occurs between the ferritic phase and the martensiticphase. Specifically, the Cr content in the ferritic phase is higher thanthe Cr content in the martensitic phase. Chromium is thought to beeffective in preventing the propagation of cracking in high-temperatureenvironments. However, when the volume ratio of ferritic phase increasesand exceeds 40%, the Cr content in the ferritic phase decreases belowthe content that is effective in preventing the propagation of crackingin high-temperature environments. Therefore, it is thought that crackingis liable to occur.

On the other hand, if X is more than −4, the volume ratio of ferriticphase is less than 10%. If the ferritic phase is too little, thepropagation of cracking cannot be restrained. The preferable range of Xis −7.7 to −4.3.

As described above, the ferritic phase distribution ratio is higher than85%. FIG. 1 shows one example of the cross section of the stainlesssteel in accordance with the present invention. The thickness of aferritic phase 5 near the surface 1 is mostly about 0.5 to 1 μm. Thelength of the ferritic phase 5 is mostly about 50 to 200 μm. In FIG. 1,since the ferritic phase distribution ratio is higher than 85%, theferritic phases 5 are distributed in the whole area under the surface 1.For this reason, the cracking occurring on the surface 1 reaches theferritic phase 5 at a position shallow from the surface 1, and thepropagation thereof is inhibited. Therefore, the SCC resistance isimproved.

If the ferritic phase distribution ratio is 85% or lower though theabove-described chemical composition, Formula (1), and Formula (2) arewithin the range according to the present invention, the ferritic phasedistribution ratio is 85% or lower. In FIG. 4 in which the ferriticphase distribution ratio is 85% or lower, the length of the ferriticphase 5 in the direction in parallel with the surface 1 is shorter thanthe length of the ferritic phase 5 in FIG. 1. The ferritic phases 5 inFIG. 4 are not distributed so widely as in FIG. 1. Therefore, thedistance in which a crack 7 reaches the ferritic phase 5 is longer thanthat in FIG. 1. As a result, stress corrosion cracking is liable tooccur.

3. Selective Elements

The chemical composition of the stainless steel for oil well inaccordance with the present invention may further contain, in place ofsome of Fe, one or more kinds selected from the group consisting of aplurality of elements described below.

V: 0.25% or less Nb: 0.25% or less Ti: 0.25% or less Zr: 0.25% or less

All of vanadium (V), niobium (Nb), titanium (Ti), and zirconium (Zr) areselective elements. These elements form carbides to improve the strengthand toughness of steel. However, if the contents of these elements aretoo high, the carbides coarsen, so that the toughness is deteriorated.Also, the corrosion resistance is deteriorated. Therefore, the V contentshould be 0.25% or less, the Nb content should be 0.25% or less, the Ticontent should be 0.25% or less, and the Zr content should be 0.25% orless. Preferably, the content of V, Nb or Zr is 0.005 to 0.25%, and theTi content is 0.05 to 0.25%. In this case, the above-described effectscan be achieved especially effectively.

The chemical composition of the stainless steel for oil well inaccordance with the present invention may further contain, in place ofsome of Fe, one or more kinds selected from the group consisting of aplurality of elements described below.

Ca: 0.005% or less Mg: 0.005% or less La: 0.005% or less Ce: 0.005% orless

All of calcium (Ca), magnesium (Mg), lanthanum (La), and cerium (Ce) areselective elements. These elements improve the hot workability of steel.However, if the contents of these elements are too high, coarse oxidesare formed, so that the corrosion resistance is deteriorated. Therefore,the content of each of these elements should be 0.005% or less.Preferably, the Ca content, the Mg content, the La content, and the Cecontent each are 0.0002 to 0.005%. In this case, the above-describedeffect can be achieved especially effectively.

Even if these selective elements are contained, the micro-structuredescribed in item 2 can be obtained.

4. Manufacturing Method

A method of manufacturing the stainless steel for oil well in accordancewith the present invention is described. If a steel stock (cast piece,billet, bloom and slab etc.) having the above-described chemicalcomposition and satisfying Formulas (1) and (2) is hot worked with apredetermined reduction of area, the micro-structure described in item 2can be obtained. Hereunder, the method of manufacturing the stainlesssteel pipe for oil well is described as one example of the stainlesssteel for oil well in accordance with the present invention.

S1: Steel Stock Preparing and Heating Step

A steel stock having the above-described chemical composition andsatisfying Formulas (1) and (2) is prepared. The steel stock may be acast piece manufactured by the round billet continuous casting process.Also, the steel stock may be a billet manufactured by hot working aningot manufactured by the ingot making process, or may be a billetmanufactured from a cast piece produced by the bloom continuous casting.The prepared steel stock is charged into a heating furnace or a soakingpit and is heated.

S2: Hot Working Step

Successively, the heated steel stock is hot worked to manufacture amaterial pipe. For example, the Mannesmann process is implemented forhot working. Specifically, the steel stock is pierced by a piercingmachine to form a material pipe. Then, the material pipe is rolled by amandrel mill or a sizing mill. For hot working, hot extrusion may beaccomplished, or forging may be performed.

At this time, hot working is performed so that the reduction of area ofthe steel stock at a steel stock temperature of 850 to 1250° C. is 50%or more. The reduction of area (%) is defined by the aforementionedFormula (3).

If the reduction of area of the steel stock at a steel stock temperatureof 850 to 1250° C. is 50% or more, a micro-structure in which a ferriticphase having a volume ratio of 10 to 40% is contained and the ferriticphase distribution ratio is higher than 85% can be obtained. On theother hand, even for the steel stock having the chemical composition ofthe present invention and satisfying Formulas (1) and (2), if thereduction of area is less than 50%, the ferritic phase distributionratio is sometimes 85% or less.

The material pipe having been hot worked is cooled to normaltemperature. The cooling method may be air cooling or may be watercooling.

S3 and S4: Quenching Step and Tempering Step

After the hot working, the material pipe is quenched and tempered sothat the 0.2% offset yield stress is 758 MPa or higher. The preferablequenching temperature is the Ac3 transformation point or higher. Thepreferable tempering temperature is the Ac1 transformation point orlower. Through the above-described steps, the stainless steel pipe inaccordance with the present invention is manufactured.

Method of Manufacturing Other Stainless Steel Products

The above is the description of the method of manufacturing a seamlessstainless steel pipe given as one example of the method of manufacturingthe stainless steel. The manufacturing method for other stainless steelproducts (for example, a steel plate, an electric resistance weldedsteel tube, and a laser welded steel tube) manufactured from thestainless steel is the same as that for the seamless stainless steelpipe. For example, a stainless steel plate is manufactured by rolling asteel stock by using a rolling mill in the hot working step.

EXAMPLES

A steel having the chemical composition given in Table 1 was melted tomanufacture a cast piece or a billet.

TABLE 1 Chemical compound: unit being mass %, balance being Fe andunavoidable impurities Value of Others Formula (1) Value of Classifi- V,Nb, Ti, Zr, Cr + Cu + Formula (2) cation Steel C Si Mn P S Cr Cu Ni MoAl N Ca, Mg, La, Ce Ni + Mo X Invention A 0.020 0.24 0.10 0.017 0.000916.96 2.48 5.03 2.55 0.045 0.0153 — 27.02 −5.88 steel B 0.010 0.25 0.080.017 0.0004 16.99 2.42 4.53 2.56 0.049 0.0065 — 26.50 −7.03 C 0.0250.24 0.17 0.018 0.0005 17.09 2.36 4.51 2.52 0.049 0.0066 — 26.48 −6.65 D0.027 0.25 0.13 0.017 0.0005 17.49 2.45 4.15 2.53 0.045 0.0110 — 26.62−7.24 E 0.024 0.25 0.05 0.016 0.0010 16.43 2.45 4.55 2.49 0.031 0.0200 —25.92 −5.49 F 0.023 0.24 0.18 0.019 0.0005 16.14 2.39 5.47 2.50 0.0490.0052 — 26.50 −4.70 G 0.020 0.23 0.03 0.018 0.0005 17.04 2.49 4.53 2.540.041 0.0069 — 26.60 −6.74 H 0.021 0.25 0.16 0.018 0.0004 17.05 2.394.41 2.52 0.051 0.0055 — 26.37 −6.85 I 0.033 0.24 0.15 0.018 0.000417.38 2.54 4.94 2.55 0.050 0.0131 — 27.41 −6.06 J 0.022 0.24 0.13 0.0180.0004 16.86 2.46 5.05 1.80 0.050 0.0080 — 26.17 −5.08 K 0.019 0.24 0.150.018 0.0004 16.92 2.48 5.03 3.18 0.050 0.0076 — 27.61 −6.77 L 0.0220.24 0.15 0.017 0.0004 16.86 2.91 5.03 2.55 0.050 0.0066 — 27.35 −5.73 M0.022 0.24 0.30 0.018 0.0004 16.86 2.48 5.03 2.55 0.050 0.0066 — 26.92−5.87 N 0.023 0.24 0.31 0.017 0.0004 16.95 2.39 4.57 2.55 0.055 0.0080 —26.46 −6.40 O 0.023 0.24 0.45 0.017 0.0004 17.11 2.39 4.58 2.55 0.0550.0084 — 26.63 −6.48 P 0.023 0.25 0.31 0.018 0.0006 17.03 2.38 4.10 2.520.054 0.0092 — 26.03 −6.89 Q 0.024 0.25 0.16 0.017 0.0015 16.13 2.424.56 2.51 0.044 0.0200 — 25.62 −5.13 R 0.023 0.25 0.18 0.017 0.000617.03 2.38 4.61 3.80 0.054 0.0155 — 27.82 −7.67 S 0.044 0.25 0.18 0.0170.0007 17.02 2.42 4.53 2.56 0.044 0.0065 — 26.53 −5.99 T 0.042 0.25 0.310.017 0.0004 16.99 2.42 4.53 2.56 0.049 0.0065 — 26.50 −5.96 U 0.0220.24 0.10 0.017 0.0009 17.01 2.48 5.03 2.55 0.045 0.0153 V: 0.05 27.07−5.88 V 0.022 0.24 0.10 0.017 0.0009 16.94 2.48 5.03 2.55 0.045 0.0153Nb: 0.06 27.00 −5.80 W 0.022 0.24 0.10 0.017 0.0009 17.51 2.48 5.03 2.550.045 0.0153 Ti: 0.11 27.57 −6.43 X 0.022 0.24 0.10 0.017 0.0009 16.942.48 5.03 2.55 0.045 0.0153 Zr: 0.05 27.00 −5.80 AA 0.022 0.24 0.100.017 0.0009 16.94 2.48 5.03 2.55 0.045 0.0153 Ca: 0.0010 27.00 −5.80 AB0.022 0.24 0.10 0.017 0.0009 16.94 2.48 5.03 2.55 0.045 0.0153 Mg:0.0013 27.00 −5.80 AC 0.022 0.24 0.10 0.017 0.0009 16.94 2.48 5.03 2.550.045 0.0153 V: 0.04, Ti: 0.09, 27.00 −5.80 Ca: 0.0010 AD 0.022 0.240.10 0.017 0.0009 16.94 2.48 5.03 2.55 0.045 0.0153 V: 0.06, Ti: 0.08,27.00 −5.80 Mg: 0.0021 AE 0.020 0.24 0.10 0.018 0.0009 17.01 2.48 5.062.53 0.040 0.0161 V: 0.05 27.08 −5.86 AF 0.008 0.23 0.18 0.018 0.000517.04 2.49 4.53 2.54 0.041 0.0070 V: 0.05 26.60 −7.02 Compar- BA 0.0340.25 0.16 0.017 0.0008 16.45 1.90 5.53 1.75 0.044 0.0190 — 25.63 −3.67ative BB 0.012 0.24 0.12 0.017 0.0004 17.81 2.50 4.08 2.67 0.044 0.0140— 27.06 −8.16 steel BC 0.021 0.26 0.31 0.016 0.0010 16.46 1.51 4.58 2.500.035 0.0210 — 25.05 −5.91 BD 0.021 0.24 0.31 0.016 0.0007 16.57 2.644.97 1.51 0.035 0.0190 — 25.69 −4.04 BE 0.060 0.25 0.01 0.016 0.000716.99 2.42 4.53 2.56 0.040 0.0065 — 26.50 −5.57 BF 0.030 0.25 0.32 0.0170.0010 14.89 1.02 6.21 2.01 0.001 0.0410 — 24.13 −1.38 BG 0.021 0.240.30 0.017 0.0004 17.56 2.50 3.42 2.55 0.041 0.0130 — 26.03 −8.08 BH0.020 0.23 0.32 0.015 0.0010 16.41 1.53 3.59 2.51 0.018 0.0210 — 24.04−6.87 BI 0.021 0.23 0.18 0.015 0.0010 16.15 1.01 6.02 2.51 0.002 0.0190V: 0.05 25.69 −4.51 * Underlined value indicates that the value is outof range of corresponding value of present invention. * X = 30(C + N) +0.5Mn + Ni + Cu/2 + 8.2 − 1.1(Cr + Mo)

Referring to Table 1, the chemical compositions of steels A to X and AAto AF were within the range of chemical composition of the presentinvention. Also, the chemical compositions of steels A to X and AA to AFsatisfied Formulas (1) and (2).

On the other hand, steels BA to BI departed from the range according tothe present invention. Specifically, the chemical compositions of steelsBA and BB were within the range according to the present invention, andalso satisfied Formula (1), but did not satisfy Formula (2). Thechemical composition of steel BC was within the range according to thepresent invention, and also satisfied Formula (2), but did not satisfyFormula (1). The Mo content of steel BD was lower than the lower limitof Mo content of the present invention. The C content of steel BEexceeded the upper limit of C content of the present invention. The Crcontent and the Cu content of steel BF were lower than the lower limitsof Cr content and Cu content of the present invention, and further didnot satisfy Formulas (1) and (2). The Ni content of steel BG was lowerthan the lower limit of Ni content of the present invention. The Nicontent of steel BH was lower than the lower limit of Ni content of thepresent invention, and further did not satisfy Formula (1). The Cucontent of steel BI was lower than the lower limit of Cu content of thepresent invention. The Ac1 transformation points of steels A to X, AA toAF, and BA to BI were within the range of 630 to 710° C., and the Ac3transformation points thereof were within the range of 720 to 780° C.

Steels A to X, steels AA to AD, steel AF, and steels BA to BI were castpieces each having a thickness of 30 mm. Also, steel AE was a solidround billet having a diameter of 191 mm. Steel S and steel AE each wereprepared in plural numbers.

Using the prepared cast pieces and slabs, stainless steel plates andstainless steel pipes of test numbers 1 to 44 given in Table 2 weremanufactured.

TABLE 2 Metal micro-structure High-temperature Ferritic AusteniticMartensitic Ferritic phase corrosion resistance Test YS Reduction phasevolume phase volume phase volume distribution Corrosion SSC number Steel(MPa) of area (%) ratio (%) ratio (%) ratio (%) ratio (%) Crack rateresistance 1 A 882 52.0 23 2 75 100 Absent <0.1 Absent 2 B 893 52.0 38 458 100 Absent <0.1 Absent 3 C 911 52.0 35 5 60 100 Absent <0.1 Absent 4D 911 57.9 39 1 60 100 Absent <0.1 Absent 5 E 835 57.9 18 0 82 100Absent <0.1 Absent 6 F 762 57.9 13 7 80 100 Absent <0.1 Absent 7 G 90157.9 30 5 65 100 Absent <0.1 Absent 8 H 911 57.9 32 2 66 100 Absent <0.1Absent 9 I 951 57.9 25 2 73 100 Absent <0.1 Absent 10 J 870 57.9 35 3 62100 Absent <0.1 Absent 11 K 882 57.9 20 4 76 100 Absent <0.1 Absent 12 L944 57.9 25 4 71 100 Absent <0.1 Absent 13 M 907 57.9 25 4 71 100 Absent<0.1 Absent 14 N 918 57.9 30 2 68 100 Absent <0.1 Absent 15 O 931 57.930 2 68 100 Absent <0.1 Absent 16 P 830 57.9 35 1 64 100 Absent <0.1Absent 17 Q 814 57.9 18 3 79 100 Absent <0.1 Absent 18 R 855 76.7 38 161 100 Absent <0.1 Absent 19 S 848 76.7 25 1 74 100 Absent <0.1 Absent20 T 805 76.7 20 6 74 100 Absent <0.1 Absent 21 U 951 76.7 22 1 77 100Absent <0.1 Absent 22 V 944 76.7 20 2 78 100 Absent <0.1 Absent 23 W 91076.7 28 0 72 100 Absent <0.1 Absent 24 X 924 80.0 20 2 78 100 Absent<0.1 Absent 25 AA 889 80.0 22 1 77 100 Absent <0.1 Absent 26 AB 869 80.022 2 76 100 Absent <0.1 Absent 27 AC 962 80.0 22 1 77 100 Absent <0.1Absent 28 AD 951 80.0 22 1 77 100 Absent <0.1 Absent 29 AF 893 52 33 562 95.2 Absent <0.1 Absent 30 AE 910 57.9 25 5 70 100 Absent <0.1 Absent31 AE 905 52.8 27 3 70 100 Absent <0.1 Absent 32 AE 876 44.2 22 5 7371.4 Present <0.1 Absent 33 S 820 40 15 3 82 71.4 Present <0.1 Absent 34S 811 30 13 1 86 61.9 Present <0.1 Absent 35 S 808 20 16 0 84 57.1Present <0.1 Absent 36 BA 848 57.9 1 3 96 47.6 Present <0.1 Present 37BB 869 57.9 70 0 30 100 Present <0.1 Absent 38 BC 816 57.9 20 0 80 100Present <0.1 Absent 39 BD 923 57.9 11 7 82 85.7 Present <0.1 Present 40BE 841 57.9 18 5 77 100 Present <0.1 Present 41 BF 905 57.9 0 0 100 0Present ≧0.1 Present 42 BG 910 57.9 62 3 35 100 Present <0.1 Present 43BH 805 57.9 33 0 67 100 Present <0.1 Present 44 BI 851 57.9 24 0 76 100Present <0.1 Absent

Manufacture of Stainless Steel Plate

Nos. 1 to 29 and Nos. 33 to 44 stainless steel plates were manufacturedas described below. The cast pieces of steels A to X, steels AA to AD,steel AF, and steels BA to BI were heated by a heating furnace. Theheated cast pieces were hot forged and hot rolled to manufacturestainless steel plates each having a thickness of 6 to 14.4 mm and awidth of 120 mm. The temperature of cast piece during hot working (hotforging and hot rolling) was 1000 to 1250° C. The reductions of areaduring hot working were as given in Table 2. The reduction of area wasdetermined based on Formula (3). The reductions of area of Nos. 33 to 35steel plates were less than 50%. The reductions of area of steel platesof other numbers were 50% or more.

The manufactured stainless steel plates were quenched. Specifically, thestainless steel plates were heated at a quenching temperature of 980 to1250° C. for 15 minutes, and then was water cooled. The quenchingtemperatures of all test numbers were not lower than the Ac3transformation point. The quenched steel plate was tempered at atemperature of 500 to 650° C. so that the 0.2% offset yield stress was758 to 966 MPa. The tempering temperatures of steels of all test numberswere not higher than Ac1 transformation point.

Manufacture of Stainless Steel Pipe

Nos. 30 to 32 stainless steel pipes were manufactured as describedbelow. After the round billet of steel AE has been heated by a heatingfurnace, hot working (including piercing using a piercing machine androlling using a mandrel mill) was performed to manufacture a stainlesssteel pipe (seamless steel pipe). At this time, the billet temperatureat the time of hot working was 950 to 1200° C. Also, the reduction ofarea at the time of hot working was as given in Table 2. The reductionof area of No. 32 stainless steel pipe was less than 50%. The reductionsof area of stainless steel pipes of other test numbers exceeded 50%. Themanufactured stainless steel pipe was quenched and tempered under thesame conditions as those of the above-described stainless steel plate sothat the 0.2% offset yield stress was 758 to 966 MPa.

Investigation of Micro-Structure and Ferritic Phase Distribution Ratio

A sample including the surface of the stainless steel plate or thestainless steel pipe was taken from an arbitrary location in thestainless steel plate or the stainless steel pipe of each test number.The sample surface corresponding to the cross section of the stainlesssteel plate or the stainless steel pipe was ground. After grinding, thesample surface was etched by using a solution in which glycerin is mixedwith aqua regia.

The area ratio of ferritic phase on the etched sample surface wasmeasured by the point counting method conforming to JISG0555. Themeasured area ratio was defined as the volume ratio of ferritic phase.The volume ratio of the retained austenitic phase was determined by theaforementioned X-ray diffraction method. It was assumed that themartensitic phase was the remaining portion of micro-structure otherthan the ferritic phase and the retained austenitic phase. Therefore,the volume ratio (%) of martensitic phase was determined based onFormula (b).

Volume ratio of martensitic phase=100−(volume ratio of ferriticphase+volume ratio of retained austenitic phase)  (b)

The determined volume ratios of ferritic phase, retained austeniticphase, and martensitic phase are given in Table 2.

Further, the ferritic phase distribution ratio was determined.Specifically, a scale shown in FIG. 2 was placed on the cross section ofsample of each test number to determine the ferritic phase distributionratio (%) defined by Formula (a). The determined ferritic phasedistribution ratio is given in Table 2.

Tensile Test

A round bar tensile test specimen was taken from the stainless steelplate and stainless steel pipe of each test number. Using this round bartensile test specimen, a tensile test was conducted. The longitudinaldirection of the round bar tensile test specimen was the rollingdirection of the stainless steel plate and the stainless steel pipe. Thediameter of the parallel portion of the round bar tensile test specimenwas 4 mm, and the length thereof was 20 mm. The tensile test wasconducted at normal temperature (25° C.)

High-Temperature Corrosion Resistance Test

A four-point bending test specimen was taken from the stainless steelplate and stainless steel pipe of each test number. The length of thespecimen was 75 mm, the width thereof was 10 mm, and the thicknessthereof was 2 mm. Each specimen was deflected by four-point bending. Atthis time, the deflection amount of each specimen was determined inconformity to ASTM G39 so that the stress applied to each specimen isequal to the 0.2% offset yield stress of each specimen.

An autoclave of 200° C. in which CO₂ of 3 MPa and H₂S of 0.001 MPa weresealed under pressure was prepared. The specimen subjected to deflectionwas immersed in NaCl aqueous solution of 25 wt % in the autoclave forone month. After one-month immersion, it was examined whether or notcracking occurred in the specimen. Specifically, the cross section ofthe specimen portion to which tensile stress was applied was observedusing an optical microscope of ×100 magnification to judge the presenceof crack. Also, the weight of specimen was measured before and after thetest. From the change of measured weight, the corrosion loss of specimenwas determined. Then, the corrosion rate (g/(m²+hr)) was determinedbased on the corrosion loss.

The test results are given in Table 2. The term “Present” in “Crack”item in “High-temperature corrosion resistance” column in Table 2indicates that a crack was confirmed by the observation using an opticalmicroscope. The term “Absent” indicates that a crack could not beconfirmed. The expression “≦0.1” in “Corrosion rate” item indicates thatthe corrosion rate was lower than 0.1 g/(m²+hr). The expression “≧0.1”indicates that the corrosion rate was not lower than 0.1 g/(m²+hr).

SSC Resistance Test At Normal Temperature

A four-point bending test specimen was taken from the steel plate ofeach test number. The length of the specimen was 75 mm, the widththereof was 10 mm, and the thickness thereof was 2 mm. Each specimen wasdeflected by four-point bending. At this time, the deflection amount ofeach specimen was determined in conformity to ASTM G39 so that thestress applied to each specimen is equal to the 0.2% offset yield stressof each specimen.

An autoclave of normal temperature (25° C.) in which CO₂ of 0.099 MPaand H₂S of 0.001 MPa were sealed was prepared. The specimen subjected todeflection was immersed in NaCl aqueous solution of 20 wt % in theautoclave for one month. After one-month immersion, it was examinedwhether or not cracking occurred in the specimen. The criterion of crackwas the same as that in the high-temperature corrosion resistance test.The test results are given in Table 2. The term “Present” in “SSCresistance” column in Table 2 indicates that a crack was confirmed bythe observation using an optical microscope. The term “Absent” indicatesthat a crack could not be confirmed.

Test Results

Referring to Table 2, the stainless steel plates and stainless steelpipes of test numbers 1 to 31 each had a chemical composition andmicro-structure within the range according to the present invention.Therefore, in the high-temperature corrosion resistance test, nocracking (SCC) occurred, and the corrosion rate was lower than 0.1g/(m²+hr). In the SSC resistance test at normal temperature as well, nocracking (SSC) occurred.

The chemical compositions of the stainless steel plates and stainlesssteel pipes of test numbers 32 to 35 were within the range according tothe present invention, and satisfied Formulas (1) and (2). However, theferritic phase distribution ratios thereof were lower than the lowerlimit of the present invention. Therefore, cracking occurred in thehigh-temperature corrosion resistance test. It is assumed that since thereductions of area of the stainless steel plates and stainless steelpipes of test numbers 32 to 35 were less than 50%, the ferritic phasedistribution ratios thereof were lower than the lower limit of thepresent invention.

For the steel plate of test number 36, the value of X exceeded the upperlimit of Formula (2), so that the volume ratio of ferritic phase wasless than 10%. Therefore, cracking occurred in the high-temperaturecorrosion resistance test and the SSC resistance test. For the steelplate of test number 37, the value of X was lower than the lower limitof Formula (2), so that the volume ratio of ferritic phase exceeded 40%.Therefore, cracking occurred in the high-temperature corrosionresistance test. The steel plate of test number 38 did not satisfyFormula (1). Therefore, cracking occurred in the high-temperaturecorrosion resistance test. The reason for this is probably that apassivation film, which prevents crack propagation, was less liable tobe formed on the surface of crack after the occurrence of crack.

For the steel plate of test number 39, the Mo content was lower than thelower limit of Mo content of the present invention. Therefore, crackingoccurred in the high-temperature corrosion resistance test and the SSCresistance test. For the steel plate of test number 40, the C contentexceeded the upper limit of C content of the present invention.Therefore, cracking occurred in the high-temperature corrosionresistance test and the SSC resistance test. For the steel plate of testnumber 41, the Cr content and the Cu content were lower than the lowerlimits of Cr content and Cu content of the present invention, andFormulas (1) and (2) were not satisfied. Therefore, cracking occurred inthe high-temperature corrosion resistance test and the SSC resistancetest, and the corrosion rate in the high-temperature corrosionresistance test was 0.1 g/(m²+hr) or higher. For the steel plate of testnumber 42, the Ni content is lower than the lower limit of Ni content ofthe present invention, and the value of X was lower than the lower limitvalue of Formula (2). Therefore, cracking occurred in thehigh-temperature corrosion resistance test and the SSC resistance test.For the steel plate of test number 43, the Ni content is lower than thelower limit of Ni content of the present invention, and Formula (1) wasnot satisfied. Therefore, cracking occurred in the high-temperaturecorrosion resistance test and the SSC resistance test. For the steelplate of test number 44, the Cu content is lower than the lower limit ofCu content of the present invention. Therefore, cracking occurred in thehigh-temperature corrosion resistance test. The reason for this isprobably that a passivation film was less liable to be formed on thesurface of crack after the occurrence of crack.

The above is the description of the embodiment of the present invention,and the above-described embodiment is merely an example for carrying outthe present invention. Therefore, the present invention is not limitedto the above-described embodiment, and the above-described embodimentcan be changed as appropriate without departing from the spirit andscope of the present invention.

INDUSTRIAL APPLICABILITY

The stainless steel for oil well in accordance with the presentinvention can be used for oil wells and gas wells. In particular, it canbe used for deep oil wells having a high-temperature environment. Forexample, it can be used for deep oil wells having a high-temperatureenvironment of 150° C. to 250° C.

1-6. (canceled)
 7. A method of manufacturing a stainless steel for oilwell, comprising: a step of heating a steel stock having a chemicalcomposition comprising, by mass percent, C: not more than 0.05%, Si: notmore than 0.5%, Mn: 0.01 to 0.5%, P: not more than 0.04%, S: not morethan 0.01%, Cr: more than 16.0 and not more than 18.0%, Ni: more than4.0 and not more than 5.6%, Mo: 1.6 to 4.0%, Cu: 1.5 to 3.0%, Al: 0.001to 0.10%, and N: not more than 0.050%, the balance being Fe andimpurities, and satisfying Formulas (1) and (2):Cr+Cu+Ni+Mo≧25.5  (1)−8≦30(C+N)+0.5Mn+Ni+Cu/2+8.2−1.1(Cr+Mo)≦−4  (2) where the content(percent by mass) of element is substituted for the symbol of element inFormulas (1) and (2); a step of hot working the steel stock so that thereduction of area of the steel stock at a steel stock temperature of 850to 1250° C. is not less than 50%; a step of heating the steel stock to atemperature not lower than Ac3 transformation point and quenching itafter the hot working; and a step of tempering the steel stock at atemperature not higher than Ad transformation point after the quenching,and the method being used for manufacturing a stainless steel having amicro-structure containing a martensitic phase and a ferritic phasehaving a volume ratio of 10 to 40%, and being such that when a pluralityof imaginary line segments, which each have a length of 50 μm in thethickness direction from the surface of the stainless steel and arearranged in a row at intervals of 10 μm over the range of 200 μm, areplaced on a cross section of the stainless steel, the ratio of thenumber of imaginary line segments crossing the ferritic phase to thetotal number of imaginary line segments is higher than 85%; and a 0.2%offset yield stress not lower than 758 MPa.
 8. The method ofmanufacturing a stainless steel for oil well according to claim 7,wherein the chemical composition further contains, in place of some ofFe, at least one kind selected from the group consisting of V: not morethan 0.25%, Nb: not more than 0.25%, Ti: not more than 0.25%, and Zr:not more than 0.25%.
 9. The method of manufacturing a stainless steelfor oil well according to claim 7, wherein the chemical compositionfurther contains, in place of some of Fe, at least one kind selectedfrom the group consisting of Ca: not more than 0.005%, Mg: not more than0.005%, La: not more than 0.005%, and Ce: not more than 0.005%.
 10. Themethod of manufacturing a stainless steel for oil well according toclaim 8, wherein the chemical composition further contains, in place ofsome of Fe, at least one kind selected from the group consisting of Ca:not more than 0.005%, Mg: not more than 0.005%, La: not more than0.005%, and Ce: not more than 0.005%.